Harmonic change occurs in most, if not all, musical instruments in varying degrees as they are played. The harmonic change in the resulting tone of the musical instrument noticeably varies with time. Many instruments exhibit this change during the onset of tonal reproduction followed by the settling down of the tone, harmonically, to what is commonly referred to as a steady state condition. Some examples of the types of instruments which exhibit such harmonic change with time include horns, bowed strings, and organ pipes. The onset or attack transients associated with an organ pipe can be very pronounced, often sounding like tuned wind or noise. In synthesizing or reproducing the sound of such instruments electronically, harmonic change with time is an important contribution to realism.
In prior electronic musical instruments, harmonic variations with time have been implemented with varying degrees of success. One attempt is described in U.S. Pat. No. 4,184,403, assigned to the assignee of the present invention, in which waveform generation is accomplished by successively and repeatedly reading out amplitude samples of a waveform stored in memory as full or half cycle representations of the waveform. The harmonic content of the voice or tone being generated is caused to change by reading from multiple memories singly but in sequence, where each memory contains a slightly different harmonic content.
An improved method of generating more accurate reproductions of the transient portions of a waveform is described in U.S. Pat. No. 4,352,312, also assigned to the assignee of the present invention. This improved method causes an interpolation between the various different harmonic structures yielding a very smooth and gradual change from one harmonic structure to another during the transient periods of the waveform. One of the major deficiencies with the foregoing methods is the arduous task of accurately determining the harmonic composition of the transient portion of the waveform at a given point in time coupled with an inherent inability of the circuitry of the electronic musical instrument to reproduce non-harmonic components of the waveform.
One such attempt at recreating a natural sound is described in U.S. Pat. No. 4,383,462. The patent describes the use of storing wave shapes in memory and retrieving them in accordance with the teachings of U.S. Pat. No. 3,515,792. While a "tone wave shape" of the attack period, for instance, may be stored in memory, the envelope characteristics are still generated separately before storing them in memory. A "complete wave shape" is a combination of the "tone wave shape" with its corresponding envelope added to it. Thus, the system perpetrates the deficiency of not being able to recreate the harmonic and non-harmonic content of the waveform because the wave shape and envelope are created separately before they are added together and stored in memory. The remaining portions of the wave form, the steady state and decay periods, are recreated from standard wave shapes and a separate envelope generator in accordance with current practice. This system provides little insight into overcoming the stated deficiencies and in some respects perpetrates them.
It is therefore an object of the present invention to eliminate the disadvantages stated above by providing a sufficiently large memory to store and retrieve the complete attack transient portions of the waveform and some number of cycles of the steady state portion of the waveform, both stored as contiguous amplitude samples, along with the proper memory read control circuitry.
It is also an object of the invention to provide the capability to reproduce the non-harmonic components of the waveform throughout its entire structure.
It is a further object of the present invention to provide a means for recirculating one or more of the steady state cycles of the waveform in order to create a sustained tone while the actuating key of the electronic musical instrument remains depressed.
Other objects will appear hereinafter.